P: Lightroom Classic 15.6 / Camera Raw 18.7 ignore RGBTables and ProfileGainTableMap2 in camera profiles

Camera profiles that contain two DNG 1.6/1.7 profile features load and can be selected, but the features have no effect on the rendered image:

• RGBTables (tag 52543 / 0xCD3F)
• ProfileGainTableMap2 (tag 52544 / 0xCD40)

This applies to external DCP profiles and to profiles embedded in DNG files, in Lightroom Classic and in Camera Raw in Photoshop. The DNG SDK 1.7.1 parses both tags in camera profiles, and its reference rendering applies them to the same files.

Test 1 – RGBTables (SDR)

• Two DCPs for the Nikon Z 7, identical except for the RGBTables payload: one with an identity table, one with a defined chroma change. Each is a group (ProfileGroupName) with an SDR member and an HDR member (ProfileDynamicRange).
• Installed in %APPDATA%\Adobe\CameraRaw\CameraProfiles, Lightroom restarted.
• Applied to the same raw file with identical neutral settings (white balance As Shot, all sliders 0, sharpening and noise reduction 0, linear tone curve), HDR editing off.
• Exported as 16-bit TIFF, ProPhoto RGB.
• Result: the decoded pixel data of both exports is identical (SHA-256 equal, 0 differing pixels). The XMP correctly records crs:CameraProfile="Group: ", so the profile is found and selected.
• For comparison, the DNG SDK 1.7.1 (dng_validate with the external profile attached) renders the same raw file with the same two DCPs clearly differently: 96.6 % of pixels differ, by up to 3412 codes (16-bit ProPhoto). The SDK reports the table digests as DE1B307D… (identity) and A5A79201… (chroma).

The exports stay pixel-identical in every variant I tried:

• Lightroom Classic 15.5.1 / ACR 18.5.1 and 15.6 / ACR 18.7; SDR and HDR; at -3, 0 and +3 EV.
• Camera Raw 18.7 in Photoshop.
• Raw file: a synthetic Nikon Z 7 test DNG marked as DNGVersion 1.4.0.0 or as 1.7.1.0, or a real Nikon Z 7 NEF. For the NEF, the profile is set by name in the XMP sidecar. The two exports record different crs:CameraProfileDigest values, so both profiles were loaded.
• Profile embedded in the DNG (RGBTables in IFD0), with an unknown camera model, so that the embedded profile is the only one available. Lightroom selects and records the embedded profile, again with different digests for the two files. The SDK renders these two DNGs differently (96.6 % of pixels).
• RGBTables stored under the draft tag 52539 (0xCD3B, the layout of tcRGBTablesDraft in the SDK, without the composite-method field).

Test 2 – ProfileGainTableMap2 (HDR)

• Two DCP groups that differ only in their gain table map: identity versus a tone change. Both members of each group carry the same map, so the result does not depend on which member is selected.
• HDR editing on, export as 16-bit TIFF in HDR Rec.2020 (PQ).
• Result: pixel-identical exports, for a real NEF (15.5.1 and 15.6) and for the test DNG (15.6).
• The DNG SDK 1.7.1 renders the same DNG with the two profiles differently: 95.3 % of pixels differ, by up to 11151 codes (16-bit ProPhoto, SDR rendering).

The tags are parsed: A variant that stores the composite-method field under the draft tag 52539, which the SDK rejects, does not appear in the profile browser. Camera Raw's profile index (CameraProfiles\Index.dat) lists that file without a model_restriction entry, unlike all other profiles. So Camera Raw reads and validates the tables but does not apply them when rendering. By contrast, RGB tables in XMP profiles (crs:RGBTable) do take effect.

Expected: The RGB table and the gain table map are applied, as in the DNG SDK 1.7.1 reference code.

Questions:

1. Are RGBTables and ProfileGainTableMap2 in camera profiles supported in Lightroom Classic and Camera Raw? If so, under which conditions? If not, is support planned?
2. If they are ignored on purpose, could this be documented? The DNG specification and the SDK treat them as part of the camera profile.

System: Windows 11 Pro (build 26200), NVIDIA GeForce RTX 5090, graphics processor: Same result with the graphics processor turned off, process version 6 (2012, 15.4).

I can provide the DCP files, the test raw files, the exports and SHA-256 hashes.